Problem 8 - A Complete Resource Book in Chemistry for JEE Main

Question

Phenol is less acidic than (a) o-nitrophenol (b) p-methylphenol (c) methanol (d) ethanol

Step-by-Step Solution

Answer

Phenol is less acidic than o-nitrophenol.

Step 1: Understanding Phenol's Acidity

Phenol is an aromatic compound with a hydroxyl group attached to the benzene ring, making it more acidic than alcohols due to resonance stabilization of its conjugate base, phenoxide ion.

Step 2: Analyze o-nitrophenol

In o-nitrophenol, the presence of the electron-withdrawing nitro group at the ortho position increases the acidity of phenol by stabilizing the conjugate base through resonance and inductive effects. Thus, o-nitrophenol is more acidic than phenol.

Step 3: Analyze p-methylphenol

p-methylphenol, also known as p-cresol, has a methyl group at the para position. Methyl is an electron-donating group, which decreases the acidity compared to phenol. Hence, p-methylphenol is less acidic than phenol.

Step 4: Analyze Methanol

Methanol is an aliphatic alcohol and lacks the resonance stabilization of a conjugate base that phenol has. Hence, methanol is less acidic than phenol.

Step 5: Analyze Ethanol

Ethanol, like methanol, is an aliphatic alcohol and is also less acidic than phenol due to the lack of resonance stabilization in its conjugate base.

Key Concepts

Resonance Stabilization

Phenol is distinguished from regular alcohols due to its aromatic nature. This aromaticity allows for resonance stabilization, which is critical for understanding its acidic properties. In phenol, the hydroxyl ( \(-OH\)) group is directly attached to a benzene ring. This connection means that the lone pair of electrons on the oxygen atom can be delocalized into the benzene ring. As a result, when phenol loses a proton ( \(H^+\)), the phenoxide ion ( \(C_6H_5O^−\)) that results is more stable, thanks to the spread of negative charge over the aromatic system. This resonance stabilization is a key factor in increasing the acidity of phenol relative to other alcohols, which lack such forms of stabilization.

Electron-Withdrawing and Donating Groups

Understanding how electron-withdrawing and donating groups affect acidity is crucial when comparing phenol to substituted phenols. Electron-withdrawing groups, like the nitro group ( \(-NO_2\)) in o-nitrophenol, enhance acidity by stabilizing the negative charge of the conjugate base through resonance and inductive effects. This increased stabilization makes it easier for the molecule to release a proton.

• Electron-Withdrawing: Groups like nitro increase acidity.
• Electron-Donating: Groups like methyl decrease acidity.

On the other hand, electron-donating groups such as the methyl group ( \(-CH_3\)) in p-methylphenol reduce acidity by making the release of the proton less favorable. They effectively destabilize the conjugate base by providing extra electron density.

Conjugate Base Stability

To understand acidity, analyzing the stability of the conjugate base is vital. For phenol, the conjugate base is the phenoxide ion. Its stability is crucial in determining the overall acidity of the parent molecule. Resonance plays a significant role here, as it allows the negative charge of the phenoxide ion to be distributed over the aromatic ring, reducing the energy of the ion, thereby stabilizing it. Furthermore, the presence of substituents can affect this stability:

• Electron-withdrawing groups stabilize the conjugate base further, increasing acidity.
• Electron-donating groups destabilize the conjugate base, decreasing acidity.

In alcohols like methanol or ethanol, the conjugate base lacks such resonance stabilization, making them less acidic compared to phenol. These alcohols form alkoxide ions upon deprotonation, which don’t benefit from the charge distribution that phenoxide enjoys.

Alcohol vs. Phenol Acid Strength

When comparing the acid strength of alcohols and phenols, phenol usually comes out as more acidic. The critical aspect lies in the presence of the aromatic ring in phenol, which facilitates resonance stabilization of its conjugate base. In contrast, alcohols like methanol and ethanol do not have aromatic rings, meaning they cannot stabilize their conjugate bases via resonance. Instead, they form less stable alkoxide ions because negative charges remain primarily localized on the oxygen atom.

• Phenols: Can delocalize the negative charge through resonance, leading to increased acidity.
• Alcohols: Lack aromatic resonance, resulting in decreased acidity.

This lack of resonance in alcohols is a pivotal reason why phenol is more acidic than both methanol and ethanol.